Images obtained by image pickup apparatuses such as digital cameras are degraded in quality because of blurring. Image blur is caused by spherical aberration, coma aberration, curvature of field, astigmatism, and the like of image pickup systems.
One known method for correcting such image blur is to correct image blur using information about the optical transfer function (OTF) of an image pickup system. This method is called the term image restoration or image recovery. The process for correcting a degraded image using information about the optical transfer function of an image pickup system is hereinafter referred to as an image restoration process.
One known image restoration filter (filter) used for the image restoration process is a Wiener filter that controls a degree of restoration in accordance with the intensity ratio of an image signal to a noise signal (SNR).
PTL 1 discloses an image restoration filter which is an application of a Wiener filter and which is capable of changing the degree of restoration of an image in a range from a filter that outputs an input image as it is to a filter that maximally performs image restoration, by adjusting an adjustment parameter α for the filter.